This invention relates to telecines, that is, apparatus designed to provide a video or television signal from cinematographic film.
The earliest telecines were based on the conventional cinematographic projector and had a pull-down mechanism to pull down the film by one frame whereupon the frame was scanned to produce a video signal. However, the output signal has to conform with video scanning standards and this meant that the film movement between scans should be accomplished during the video field blanking interval of typically 1.6 milliseconds. While some such fast pull-down intermittent motion telecines have been produced, in fact this short time period is not sufficient to allow the mechanism to move the film with the desired accuracy.
This problem is overcome in the continuous motion flying-spot telecine which images a cathode ray tube raster onto the film while the film moves steadily past it. Thus part of the vertical scanning action is produced by the motion of the film, and the height of the cathode ray tube raster is reduced. Typically at normal operating speed the raster height may be only about 10% of its normal stationary height. This has several deleterious effects. The image of the phosphor grain particles on the tube face is stretched in the vertical plane, and a similar effect occurs with dust particles in certain parts of the optical system, making the defects they cause more visible. The different raster heights can cause burn patches on the cathode ray tube face which require compensation if the appearance of light and dark bands across the picture is to be avoided. Indeed at certain combinations of film speed and desired scan rate the raster height becomes zero on the tube face, resulting in a worsening of these problems. Dust and phosphor particles appear as vertical lines the full picture height, severe damage is caused to the cathode ray tube phosphor and changes in the light output and colour of the cathode ray tube phosphor can result.
A further difficulty with continuous motion film scanning is the very small steadiness errors caused by scanning the film when it is in motion; these errors become very significant in video editing techniques where it is required to overlay the telecine video with video from another source. It is known that intermittent motion projectors using register pins to locate the film and hold it steady during scanning are capable of producing much improved picture steadiness. However, this is conditional on the speed being reduced so as to allow sufficient film pull-down time.